Flow property measurement using the Jenike shear cell for 7 different bulk solids

Flow property measurement using the Jenike shear cell for 7 different bulk solids Proceedings of European Congress of Chemical Engineering (ECCE-6) Co...
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Flow property measurement using the Jenike shear cell for 7 different bulk solids Proceedings of European Congress of Chemical Engineering (ECCE-6) Copenhagen, 16-20 September 2007

Flow property measurement using the Jenike shear cell for 7 different bulk solids A. Serkan ÇA LIa, B. Nuran DEVEC a, C. Hasancan OKUTANa, D. A. Ayhan RKEC b, E. Y ld m TEOMANc a

Department of Chemical Engineering, Istanbul Technical University, 34469, Maslak, Istanbul, Turkey Department of Mining Engineering, Istanbul Technical University, 34469, Maslak, Istanbul, Turkey c Türkiye Sise ve Cam Fab. A. S., Vice Pres. Res.&Tech., 34330, 4. Levent, Istanbul, Turkey b

Abstract It is essential to understand the effect of material characteristics such as size and moisture content on the flow of bulk solids. In this study, instantaneous flow properties (cohesion, angle of internal friction) of 3 different sand samples (Yal köy, Safaalan and yellow), soda, limestone, dolomite and clay were measured at intrinsic moisture contents by using Jenike shear tester. Additional tests were also performed for low and high moisture contents of Yal köy sand. Flowability of solids which is characterized by flow function (or flow index) was investigated for studied materials and a classification was made. It has been found that cohesion values increase with increasing normal stress values and high moisture content and small particle size also increase the cohesion values. Beside, there is not significant difference between the angle of internal friction values at different stress levels. The material which has the hardest flow is clay and soda has the easiest flow among the studied materials. While clay and soda have been classified as cohesive and free flowing material, all other materials have been found in easy flowing material class. It can be said that high moisture content and small particle size has an adverse effect on the flow. While the flow of Yal köy sand at low moisture content has been easier than its intrinsic moisture content, a more difficult flow hasn’t been obtained at high moisture content as expected. Keywords: Shear testing, yield locus, Jenike shear tester, flow properties, flow function 1. Introduction Of all the materials handled and produced by the chemical and process industries are in the solid state and always in particulate form. In the chemical industry alone the quantity of the product in the particulate form is greater than 30% of the whole. These

A. Serkan Cagli et al.

materials are ranging from agrochemicals to pigments, from detergents to foods, from plastics to pharmaceuticals. The handling (storage, transportation and charging) of these materials is an important operation in these industries. With the increase in the need to store bulk solids, reliable flow from storage devices has become more important than ever before. However, surveys on the performance of the processes designed to produce particulate products have shown inadequate design and poor system reliability. From an energy point of view, the handling operations have performed wastefully and improvements in techniques could lead to considerable savings over a wide range of industries [1]. Some materials flow better than others and it is well known that wet and fine solid materials flow poorly and cause obstructions in the flow. Therefore, it is essential to understand the effect of material characteristics such as size, composition and moisture content on the static and dynamic behavior of solid materials in silos (bunker, bin). Determining the flow properites of bulk solids is the first mandatory step of silo design procedure and it is important for a proper and efficient bin design. The standard method to characterize flow properties of solid materials is the shear testing which provides the information for the yield locus of the solid in question. All of the other flow properties (angle of internal friction, cohesion, flow function, kinematic angle of wall friction, etc.) of solids are also determined from the yield loci. Shear testing is based on the information of shear stress values against normal stress values and these are obtained by sliding the material inside itself under defined load values. The aim of this study is to determine the flow properties of some raw materials used in glass industry by shear tests and to understand the effect of parameters such as moisture content and mean particle size on the flow of materials. 2. Theory 2.1. Yield Locus The shear stress ( ) generated along a defined plane depends on the normal stress ( ) exerted on this plane. If a material is subjected to a shearing action, a characteristic relation is obtained between normal and shear stresses for each material. This relationship is graphically shown in coordinates (namely Mohr diagrams) and the curve - always a straight line- obtained finally is the yield locus for a bulk material. All of the flow parameters of bulk materials are obtained from these yield loci [2]. A yield locus is an important tool in determining the flow properties of bulk materials. There are generally two types of bulk materials such as free flowing and cohesive in relation to their flow properties. It can be seen from the form and position of yield locus in the Mohr diagram which type of material is. A cohesive bulk material is named as Coulomb solid and expressed by a linear yield locus in soil mechanics:

τ = tan φσ + C

(1)

2

Flow property measurement using the Jenike shear cell for 7 different bulk solids

is the shear stress, is the normal stress, C is the cohesion and φ is the angle of the internal friction of the material. φ is an indication of the friction coefficient within the material. It has a constant value as it changes at low consolidation stress levels in the case of curved yield locus [3]. A yield locus for a cohesive solid is shown in Figure 1.

Fig. 1. Yield locus for a cohesive solid and related parameters [3]

In order to initiate the motion within the solid body (flow-plastic deformation) at least one point on the Mohr circle should correspond to a failure plane. The location of the failure plane on the Mohr circle is obtained by the tangency of the material yield locus to the Mohr circle. From this point of view, Mohr circles have an important role in defining characteristic properties of bulk materials using yield locus [3, 4]. 2.2. Flow Function Major principal stress in the steady state flow is called major consolidation stress ( 1). 1, acting on critical consolidation condition, is determined by drawing the Mohr circle (steady state Mohr circle) passing through the point ( c, c) which represents the consolidation conditions in shear tests (Figure 1). The circle is tangent to the yield locus and the intersection of circle with normal stress axis gives 1 value. Unconfined yield stress ( c) is the maximum normal stress value which a solid having a free and stressless surface flows or deforms. While yield locus of a solid is known, c is found by drawing a Mohr circle (unconfined yield stress Mohr circle) tangent to the yield locus and passing through the orijin ( =0). There is a corresponding value of c for each consolidation stress ( 1), c increases as consolidation stress increases. If c values are plotted against 1 values, flow function (FF) of the material is obtained and it characterises the flow capability of a bulk material [3, 5, 6].

3

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A straight line approach can be made for most material’s flow function [6, 7]. Material cohesiveness shows increase with inreasing slope values regarding flow function graph, decrease in slope values illustrates a easier flow of solid (Figure 2). As the flow functions having different slopes can belong to different materials, it also represents the same solid at different moisture contents. The flow index (ffc), defined as the inverse slope of the flow function (FF), is used to classify powder flowability with higher values representing an easier flow [7]. This classification is given in Table 1.

Fig. 2. Solid propety according to FF slope Solids may contain both fine and coarse particles in different sieve sizes, therefore the flow properties of fine fraction of the solid is always dominant in solids flowability. This situation can be explained by the truth that shearing action occurs through the fine particles within the solid [5]. Hence, the higher the fine particles ratio in the solid, namely the smaller the mean particle size of the solid, the higher the shear stresses obtained with these solids under same loads. This means that the flow of the material will be difficult. The fine particles have also more impact in solids cohesion than the coarse particles do. So, the higher the fine particles fraction in the solid, the higher the cohesion values obtained. Moisture content may make a material cohesive and the flow may happen in a difficult way. As the moisture content increases, the cohesion values increase depending on the increasing capiler forces between the particles.

Table 1. The classification of powder flowability by flow index (ffc) [7] Flowability Flow index (ffc)

Hardened

Very cohesive

Cohesive

Easy flowing

Free flowing

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